1. Field of the Invention
The invention relates to a process for optimization of mechanical inner ear stimulation with an at least partially implantable hearing system for rehabilitation of a hearing impairment. In particular, the invention relates to a process of this type in which the hearing system has an electromechanical converter which transmits its output-side mechanical vibrational energy, via mechanical stimulation of a middle ear ossicle, to a damaged inner ear.
2. Description of Related Art
Electronic measures for rehabilitation of inner ear damage which cannot be cured by surgery have currently achieved great importance. With total failure of the inner ear, cochlear implants with direct electrical stimulation of the remaining auditory nerves are in routine clinical use. For medium to severe inner ear damage, for the first time, fully digital hearing devices are presently being used which open up a new world of electronic audio signal processing and offer expanded possibilities of controlled audiological fine tuning of the hearing devices to the individual inner ear damage. In spite of major improvements of hearing aid hardware achieved in recent years, in conventional hearing aids, there remain basic defects which are caused by the principle of acoustic amplification, i.e. especially by the reconversion of the electronically amplified signals in airborne sound. These defects include aspects such as the visibility of the hearing aids, poor sound quality as a result of electromagnetic converters (speakers), closed external auditory canal as well as feedback effects with high acoustic gain.
As a result of these fundamental defects, there has long been the desire to move away from conventional hearing aids with acoustic stimulation of the damaged inner ear and to replace them by implants with direct mechanical stimulation. Implantable hearing aids differ from conventional hearing aids: the acoustic signal is converted with a proper microphone into an electrical signal and amplified in an electronic signal processing stage; this amplified electrical signal, however, is not sent to an electroacoustic converter (speaker), but to an implanted electromechanical converter with output-side mechanical vibrations which are sent directly, therefore with direct mechanical contact, to the middle ear or inner ear, or indirectly via an air gap in, for example, electromagnetic converter systems. This principle applies regardless of whether implantation of all necessary system elements is partial or complete and also regardless of whether an individual with pure inner ear impairment with a completely intact middle ear or an individual with combined hearing impairment, in which the middle and inner ear is damaged, is to be rehabilitated.
Electromechanical converter processes include basically all physical conversion principles, such as electromagnetic, electrodynamic, magnetostrictive, dielectric and piezoelectric. Various research groups, in recent years, have focused essentially on two of these processes, specifically electromagnetic and piezoelectric processes. A survey can be found in ZENNER and LEYSIEFFER (HNO October 1997, pp. 749-774).
In the piezoelectric process, direct mechanical coupling of the output-side converter vibrations to the middle ear ossicle or to the oval window is essential. In the electromagnetic principle, force coupling between the converter and ossicle, on the one hand, can take place "without contact", i.e. via an air gap; in this case, only the permanent magnet is caused to vibrate by the converter being in direct mechanical contact with the middle ear ossicle by permanent fixation. On the other hand, it is possible to implement the converter entirely in a housing (in this case the coil and the magnet preferably being coupled with the smallest possible air gap) and to transmit the output-side vibrations via a mechanically stiff coupling element with direct contact to the middle ear ossicle (see FREDRICKSON et al.: Ongoing investigations into an implantable electromagnetic hearing aid for moderate to severe sensorineural hearing loss; Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp. 107-121; and Leysieffer et al., HNO October 1997, pp. 792-800).
The patent literature contains some of the aforementioned versions of both electromagnetic and also piezoelectric hearing aid converters: U.S. Pat. No. 3,712,962, EPLEY; U.S. Pat. No. 3,870,832, FREDRICKSON; U.S. Pat. No. 3,882,285, NUNLEY; U.S. Pat. No. 5,277,694, LEYSIEFFER et al.; U.S. Pat. No. 5,015,224, MANIGLIA; U.S. Pat. No. 4,850,962, SCHAEFER; U.S. Pat. No. 5,554,096, BALL.
The partially implantable piezoelectric hearing system of the Japanese group of Suzuki and Yanigahara presupposes, for implantation of the converter, the absence of a middle ear ossicle and a free tympanic cavity to be able to couple the piezoelement to the stapes (Yanigahara et al.: Efficacy of the partially implantable middle ear implant in middle and inner ear disorders: Adv. Audiol., Vol. 4, Karger Basel (1988), pp. 149-159, Suzuki et al.: Implantation of partially implantable middle ear implant and the indication. Adv. Audiol., Vol. 4, Karger Basel (1988), pp. 160-166). Likewise, in the method of implanting a hearing system for inner ear hearing-impaired according to SCHAEFER (U.S. Pat. No. 4,850,962) basically the incus is removed in order to be able to couple a piezoelectric converter element to the stapes.
The BALL electromagnetic converter ("Floating Mass Transducer FMT" of U.S. Pat. No. 5,554,096) is, on the other hand, fixed directly to the long process of the incus when the middle ear is intact. The electromagnetic converter of the partially implantable system of FREDRICKSON (Fredrickson et al.: Ongoing investigations into an implantable electromagnetic hearing aid for moderate to severe sensorineural hearing loss, Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp.107-121) is mechanically coupled directly to the body of the incus when the ossicle chain of the middle ear is likewise intact. The same applies to the piezoelectric converter of LEYSIEFFER (LEYSIEFFER et al.: An implantable piezoelectric hearing aid converter for the inner ear hearing-impaired. HNO 1997/45, pp. 792-800). Also in the electromagnetic converter system of MANIGLIA (MANIGLIA et al.: Contactless semi-implantable electromagnetic middle ear device for the treatment of sensorineural hearing loss, Otolaryngologic Clinics of North America, Vol. 28/1 (1995), pp. 121-141) with the ossicular chain intact a permanent magnet is permanently mechanically fixed to the ossicular chain, but is mechanically driven via an air gap coupling by a coil.
In these latter converter systems of partially or fully implantable hearing systems in which the ossicular chain of the middle ear remains unchanged, there is the disadvantage that the mechanical vibration energy which is supplied to the ossicular chain by the electromechanical transducer is divided: one part goes as desired to the inner ear, the other part is transmitted via the existing coupling to the hammer (malleus), and thus, emitted by the eardrum as acoustic sound energy to the outside into the external auditory canal, and is thus, not available for stimulation of the damaged inner ear.